This invention relates to containment vessels for nuclear reactors, and more particularly to a containment vessel for a nuclear reactor which is adapted to suppress a sudden increase in the pressure applied to the pressure suppressing chamber by air bubbles formed in the coolant within the pressure suppressing chamber at the initial stages of occurrence of an accident involving the escape of coolant from the nuclear reactor.
Generally, a containment vessel for a boiling-water reactor is known which comprises a dry well for mounting therein a pressure vessel for the reactor, a pressure suppressing chamber having a pool of coolant therein, and a vent pipe device maintaining the dry well in communication with the coolant within the pressure suppressing chamber whereby steam of high temperature and high pressure generated by an accident involving the escape of the coolant from the reactor can be released from the dry well into the coolant within the pressure suppressing chamber. The vent pipe device comprises a plurality of vent pipe members inserted in the coolant within the pressure suppressing chamber and each having at least one exhaust port opening in the coolant. The vent pipe members are constructed and arranged such that, even if the pressure in the dry well becomes lower than the pressure in a space formed above the liquid level of the coolant within the pressure suppressing chamber, the exhaust ports of the vent pipe members are immersed in the coolant within the pressure suppressing chamber at all times. This type of containment vessel for a nuclear reactor is disclosed, for example, in the Japanese Patent Gazette of Laid-Open Patent Application No. 43091/74 of Apr. 23, 1974, issued for an application filed by Hitachi Ltd., to which the present invention has been assigned.
In the event that an accident involving the escape of coolant from the pressure vessel of a reactor occurs in the aforementioned nuclear reactor containment vessel due to a failures of the piping system or some other trouble the space within the dry well will be filled with steam of high temperature and high pressure. Such steam will be caused to pass through the vent pipe members and to be released through the exhaust ports into the coolant or cooling water within the pressure suppressing chamber so that the steam may be condensed. This permits a rise in the pressure in the dry well to be avoided. However, in the initial stages of occurrence of an accident involving the escape of coolant from the nuclear reactor, non-condensable gas or air existing in the space in the dry well will be first released under high pressure into the cooling water in the pressure suppressing chamber through the vent pipe members. Therefore, a high pressure will be suddenly applied, though transiently, to the pressure suppressing chamber by bubble formation in the cooling water. This phenomenon occurs in about 0.3 to 0.7 second after the occurrence of the accident causing the escape of coolant from the nuclear reactor. There is the danger of the pressure suppressing chamber being damaged if a high pressure is suddenly applied thereto, even if the phenomenon is transient, thereby reducing the safety of the nuclear reactor containment vessel.